Facile Design of a Soft-Tough Asymmetric Composite Electrolyte for Stable All-Solid-State Sodium Batteries

All-solid-state sodium metal batteries (ASSMBs) that employ solid polymer electrolytes (SPEs) are seen as a promising choice for next-generation, high-performance energy storage. Nevertheless, challenges such as sodium dendrite formation and poor interfacial stability between SPEs and electrodes significantly hinder their commercialization. Herein, we report a soft-tough asymmetric composite electrolyte (STCE), which integrates a ceramic-rich phase on the anode side and a polymer-rich phase on the cathode side via the spontaneous settlement of the metal–organic framework (MOF) in the polymer matrix. In this unique structure, the rigid MOF-rich phase adjacent to the sodium metal effectively suppresses dendrite formation, while the soft polymer-rich phase ensures intimate contact with the cathode to increase the interfacial compatibility. Consequently, the STCE achieves a high ionic conductivity of 5.23 × 10^–4 S cm^–1, along with significantly enhanced mechanical properties and favorable electrode/electrolyte interfaces. The Na/Na symmetric cell assembled with this STCE permits stable cycling for over 350 h with a minimal charge voltage polarization of 0.15 V, and considerable electrochemical performance is further validated in the ASSMBs. This study proposes an effective approach to designing high-performance SPEs for advanced all-solid-state batteries.